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Relationship between Voice Coil Fill Factor and Loudspeaker Efficiency

For decades voice coils have been designed for a nominal resistance of 3 to 8 ohms, despite the fact that modern audio amplifiers using switch-mode technology can be easily handle much lower impedance loads. A thorough analysis of loudspeaker efficiency showed that the efficiency can be expressed as a function of the voice coil fill factor and the geometry of the magnet system. In addition, high mass ratios are more beneficial for the efficiency of drivers using high fill factor voice coils than drivers with low mass ratio. Different voice coil winding strategies are described and their fill factors analyzed. It is found that by lowering the nominal resistance of a voice coil using rectangular wire, one can increase the fill factor. A higher fill factor will shift the low frequency upward, resulting in higher -3 dB cut-off frequencies. By using rectangular wire with low DC resistance, the fill factor could be significantly increased. The fill factor of a conventional 4-ohm voice coil was measured to be 53%.

JAES Volume 64 Issue 4 pp. 241-252; April 2016
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AES - Audio Engineering Society